This is a lovely block. If you make the outer blocks 1 1/2" finished and the diamonds and center square 3" finished, it should give you a 12" block finished. Let us know if it works.

I just located this wonderful scale for enlarging and reducing. I was looking for another pattern in my binder, and found it - I'd been looking for it too. It may help make different sizes from your favorite pattern.
http://www.thequiltedsnail.com/scale